The photographic method of obtaining direct positive images without the need for a reversal process or negative films is well known.
The practical methods which can be used to form positive images with conventional direct positive silver halide photographic photosensitive materials can be divided into two main types, excluding special cases.
The methods of the first type involve the use of prefogged silver halide emulsions. The direct positive image is obtained after development by breaking down the fogging nuclei (latent image) in the exposed parts using, for example, solarization or the Herschel effect.
The methods of the second type involve the use of internal latent image type silver halide emulsions which have not been pre-fogged. The direct positive image is obtained by means of a surface development after performing a post-imagewise exposure fogging treatment or while performing such a fogging treatment.
The internal latent image type silver halide photographic emulsions referred to above are emulsions of a type in which the photosensitive nuclei are present, in the main, within the silver halide grains and with which the latent image formed by exposure to light is formed mainly within the grains.
Silver halide emulsion grains which have a core/shell structure comprising at least two layers are known as internal latent image type silver halide photographic emulsions of the latter type. They are called core/shell type silver halide emulsions.
Of the two types of method mentioned above, those of the latter type generally provide a higher photographic speed than those of the former type and they are suitable for use in applications when high photographic speeds are required. The present invention is concerned with methods of this latter type.
Various techniques are known in this field of technology. For example, the principal techniques have been disclosed in the specifications of U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, and British Patents 1,151,363, 1,150,553 and 1.011,062.
It has been possible to make comparatively high speed photographic photosensitive materials of the direct positive type using these known methods.
Furthermore, details of the mechanism by which direct positive images are formed have been disclosed, for example, in The Theory of the Photographic Process, by T. H. James, fourth edition, chapter 7, pages 182-193, and in U.S. Pat. No. 3,761,276.
Thus, it is thought that a photographic image (i.e., direct positive image) is formed in the unexposed parts by forming fog nuclei selectively only on the surfaces of the silver halide grains in the unexposed parts on the basis of the surface desensitizing action originating from the so-called internal latent image. This image is produced within the silver halide by means of an initial imagewise exposure and then subjecting the material to an ordinary so-called surface development process.
Methods in which the whole surface of the photosensitive layer is given a second exposure, known generally as "light fogging methods" (for example, British Patent 1,151,363) and methods in which nucleating agents are used, known as "chemical fogging methods", are known as means of producing fog nuclei selectively in the way described above. The latter method has been described, for example, in Research Disclosure, volume 151, No. 15162, pages 76-78 (published November 1976).
The internal latent image type silver halide photosensitive material is subjected to a surface color development treatment after a fogging treatment has been performed or while a fogging treatment is being performed. The material is then bleached and fixed (or bleach-fixed) to provide a direct positive color image. The material is normally subjected to a water washing process and/or a stabilization process after the bleaching and fixing processes.
However, when direct positive images are formed using the chemical fogging method, the rate of development is slower than that in the case of an ordinary negative type material and the processing time is prolonged. Consequently, in the past the pH and/or the temperature of the development bath has been raised to shorten the processing time. However, there is a problem in that the minimum image density of the direct positive image obtained generally increases as the pH is raised.
On the other hand, there are various technical problems with the light fogging method since it is used for various purposes in the field of photography. That is to say, with the light fogging method the appropriate exposure brightness and the level of exposure differ according to the type and characteristics of the silver halide which is used since this method is based upon the formation of fog nuclei by the photodegradation of silver halide. Consequently, it is difficult to obtain a constant level of performance and, moreover, there is a further disadvantage in that the development apparatus is complicated and expensive. Furthermore, the development rate is still unsatisfactory.
Furthermore, a surface chemical sensitization process can be carried out with the internal latent image type silver halide emulsions, and especially with the core/shell type silver halide emulsions, in order to increase the maximum density of the direct positive image which is obtained. However, chemical sensitization to increase the maximum image density often results in an increase in the minimum image density as well. Consequently, surface chemical sensitization usually has to be stopped at an appropriate level. In that case the chemically sensitized nuclei formed on the surface are weaker than those obtained with a normal negative type material. There is the further disadvantage that the ageing stability is adversely affected to a considerable degree and so this technique is of limited value. Hence, there has been a need to reduce the fog level of core/shell type silver halide emulsions.
A pronounced increase in minimum density resulting from slight changes in conditions (especially changes in the state of agitation of the system) as mentioned above is a particular problem with core/shell type silver halide emulsions. This causes major problems when scaling up for production and with repeat reproducibility because of the increase in minimum image density. Furthermore, the increase in minimum image density is sometimes such that the minimum image density increases further after long term ageing, and performance as a photosensitive material is adversely affected to a considerable degree.
The addition of stabilizers such as 4-hydroxy-6-methyl-1,3,3a,7-tetra-azaindene and 1-phenyl-5-mercaptotetrazole for example, compounds well known in the past for improving material with respect to the problems of increasing minimum image densities and variation in performance due to ageing, has been investigated. But large amounts of these compounds must be added in order to prevent any change in performance due to ageing, and it is therefore impossible to avoid having some adverse effect on the photographic performance obtained. In practical terms, these adverse effects include a lowering of the maximum density of the positive image due to a development inhibiting action and an increase in re-reversal image speed. Moreover, when spectral sensitization is carried out in the usual way with the silver halide emulsions, these compounds inhibit the adsorption of the sensitizing dyes and have an adverse effect on the increase in speed due to spectral sensitization. Therefore, discovery of technique for improving ageing stability while avoiding these adverse effects is clearly desirable.
Moreover, pressure is sometimes applied to the emulsions of photographic photosensitive materials by contact with the machine during the various processes from exposure through development processing. This pressure can result in a change in performance after processing due to pressure sensitization and desensitization. Improvement in resistance to such pressure is also desirable for core/shell internal latent image type silver halide emulsions.